Display systems, such as liquid crystal display (LCD) systems, are used in a variety of applications and commercially available devices such as, for example, computer monitors, personal digital assistants (PDAs), mobile phones, miniature music players, and thin LCD televisions. Most LCDs include a liquid crystal panel and an extended area light source, often referred to as a backlight, for illuminating the liquid crystal panel. Backlights typically include one or more lamps and a number of light management films such as, for example, light guides, mirror films, light redirecting films (including brightness enhancement films), retarder films, light polarizing films, and diffuser films. Diffuser films are typically included to hide optical defects and improve the brightness uniformity of the light emitted by the backlight.
Some diffusing films use a beaded construction to provide the light diffusion. For example, an optical film may have a layer of microscopic beads adhered to one surface of the film, and the refraction of light at the bead surfaces may operate to provide the light diffusion characteristics of the film. Examples of beaded diffusing films include: a linear prismatic brightness enhancement film with a matte surface of sparsely distributed beads, sold under the product designation TBEF2-GM by 3M Company, referred to herein as a “sparsely distributed beaded diffuser” or “SDB diffuser”; a reflective polarizing film with a beaded diffuser layer, sold under the product designation DBEF-D3-340 by 3M Company, referred to herein as a “densely-packed beaded diffuser” or “DPB diffuser”; and a diffusing cover sheet included in a commercial display device, referred to herein as a “commercial cover sheet diffuser” or “CCS diffuser”. FIG. 1 shows a scanning electron microscope (SEM) image of a representative portion of the beaded surface of a CCS diffuser, and FIG. 1A shows an SEM image of such surface in cross-section. FIGS. 2 and 3 show SEM images of representative portions of a DPB diffuser and a SDB diffuser, respectively.
Other diffusing films use a structured surface other than a beaded layer to provide the light diffusion, where the structured surface is made by microreplication from a structured tool. Examples of such diffusing films include: films (referred to herein as “Type I Microreplicated” diffusing films) with rounded or curved structures microreplicated from a tool having corresponding structures made by removing material from the tool with a cutter, as described in US 2012/0113622 (Aronson et al.), US 2012/0147593 (Yapel et al.), WO 2011/056475 (Barbie), and WO 2012/0141261 (Aronson et al.); and films (referred to herein at “Type II Microreplicated” diffusing films) with flat-faceted structures microreplicated from a tool having corresponding structures made by an electroplating process, as described in US 2010/0302479 (Aronson et al.). An SEM image of a representative portion of the structured surface of a Type I Microreplicated diffusing film is shown in FIG. 4, and a similar image of a Type II Microreplicated diffusing film is shown in FIG. 5. Still other microreplicated diffusing films include films in which a tool surface is made to be structured by a sandblasting procedure, and the structured surface is then imparted to the film by microreplication from the tool. See e.g. U.S. Pat. No. 7,480,097 (Nagahama et al.).